Optical discs have become increasingly popular as an efficient and cost-effective storage medium for digitally stored data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands (also sometimes referred to as “marks” and “spaces”) along a continuously extending spiral track. The length of each pit and land corresponds to one of a selected number of data symbols (for example, from 3T to 11T, with T of determined length).
The data symbols are recovered from the disc through the use of a light source (such as a laser) which applies light of selected wavelength to the rotating disc and a transducer which generates a readback signal indicative of the data in relation to relative differences in reflectivity of the pits and lands. It is common to separate the relative elevations of the pits and the lands by a distance equal to a quarter-wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands. Optical disc mastering is discussed, for example, by Published PCT Document WO 99/44196 (PCT Patent Application PCT/US99/04338), assigned to the assignee of the present invention and incorporated herein by reference.
One popular optical disc format is commonly referred to as compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CDs). A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 650 megabytes (MB).
Another popular optical disc format is commonly referred to as digital versatile disc, or DVD. A DVD can be considered a “high-density” CD, in that a typical DVD has generally the same dimensions as a CD, but can store about 4.7 gigabytes (GB) of data per recording layer, due to increased data storage densities through reductions in pit/land geometries and improvements in data encoding and recovery techniques. Accordingly, DVDs can be advantageously utilized as a storage medium for full-length movies (video DVD), computer software (DVD-ROM) and music (audio DVD).
Due to the worldwide consumer demand for the types of information available on optical discs (e.g., software, music and movies), combined with the relative ease with which unauthorized copies of optical discs can be generated, suppliers of optical discs have attempted to implement various copy protection schemes to restrict unauthorized replication of the discs. One type of copy protection involves configuring an optical disc in such a manner so that an authorized copy functions properly in a readback system, but an unauthorized copy is prevented from doing so. Another type of copy protection provides a forensic tracking scheme by storing certain “marking” or “tracking” information on the disc, with this information relating to the source of the disc, the mastering date, and so on. The forensic information does not generally prevent an unauthorized copy from functioning in the readback system, but based on the presence or absence of the information, a determination can be made whether a particular optical disc is in fact an authorized copy.
One particularly common copy protection scheme for CDs involves manipulation of the error correction system. As will be recognized by those skilled in the art, CDs generally utilize three levels of error correction, referred to as C1, C2 and Level 3. The C1 codes are parity words determined from the user data, the C2 codes are error correction codes (ECC, such as Reed-Solomon codes) calculated from the user data and the C1 codes, and Level 3 are top level parity words. These levels of error detection and correction codes are calculated and stored as the user data are written to the discs and each provide successively higher levels of error detection and correction capabilities for the associated user data; if errors cannot be corrected from the C1 codes, the C2 codes are employed, and if the C2 codes are unsuccessful, then the Level 3 parity words are used to correct the errors.
Purposefully writing erroneous (“bad”) C2 codes at a few selected locations on the disc will result in a particular disc configuration that would not normally occur during normal C2 calculation steps. Thus, if the optical disc is a CD-ROM, an active application resident in a computer can first check the CD-ROM to ensure that the bad C2 codes are present at the selected locations to verify that the CD-ROM is an authorized copy. If the C2 codes are not bad at the selected locations, the application can report an error and prevent the user from using the CD-ROM.
While operable, there are disadvantages with this type of copy protection approach. Manipulating the error correction system results in some degradation of the error correcting capabilities of the disc, at least potentially making the disc unusable if enough errors arise over time that cannot be adequately corrected using the C1 and Level 3 codes.
Such manipulation also results in optical discs that technically do not meet the various CD industry format standards, and accordingly, the bad C2 codes are easily detectable by standard test equipment used to verify optical disc replicas. Thus, during manufacturing by an authorized replication source the detected errors will be reported for every replicated disc. These reported errors will have to be investigated to determine whether the errors are valid errors, or errors due to the copy protection scheme (and should thus be ignored).
More importantly, because the bad C2 codes are easily detected, an unauthorized source can easily insert bad C2 codes in the appropriate locations in the master disc and defeat the copy protection scheme altogether. Further, since audio CDs do not use a resident computer application that can initially verify the authenticity of the disc, this type of copy protection scheme cannot be readily applied to audio CDs.
DVDs do not use the three level (C1, C2 and Level 3) error correction scheme of CDs; rather, DVDs use a more efficient, two-dimensional (PI/PO) parity calculation scheme to correct readback data errors. While advantageously reducing the amount of disc space required for the error correction scheme (and thereby increasing DVD data storage capacity), it will be recognized that the bad C2 copy protection approach used with CD-ROMs cannot be used with DVDs to provide copy protection. Instead, DVD manufacturers presently use a complex, proprietary copy protection scheme referred to as the Content Scramble System, or CSS.
Different CSS schemes are used for video and audio applications (referred to respectively as “CSS1” and “CSS2”). To date, a CSS scheme has not yet been introduced for DVD-ROM, as CSS is primarily designed to scramble video/audio formatted data, and has not been applied to textual/database formatted data. Basically, CSS uses a variety of keys to scramble and descramble the user data in selected user data blocks. The keys are generated from selected bits stored at various locations on the disc, and these keys are prohibited from being output onto a computer bus, either during the mastering process or during playback of a replicated DVD. Under the current scheme, it is difficult for mastering facilities or replication sources to apply forensic tracking information to replicated DVDs.
Because of these and other considerations, there remains a continued need in the art for an improved copy protection/forensic tracking scheme that can readily be used on a variety of different types of optical discs (including the various CD and DVD audio, software and video formats) in conjunction with, and without interfering with, the operation of existing copy protection schemes (such as CSS). It is to such improvements that the present invention is directed.